I would go with linear wound springs. I don't think the initial softness of a progressive spring in a telescoping fork tube for going over minor road imperfections is advantageous to a sidecar rig. I think increased linear firmness is what you would need in hot left turning or braking situation, assuming right-sidecar. A rig is much more likely to flip back over front in a left turn when under-sprung as the tub can catch the road surface.